Apparatus for ascertaining optimum values for tube current and anode voltage of x-ray tubes



Jan. 27, 1970 S.BRANDEL1K ETAL 3,492,483

APPARATUS FOR ASCERTAINING OPTIMUM VALUES FDR TUBE CURRENT AND ANODE VOLTAGE OF X-RAY TUBES Filed March 24, 1%?

o.c. 2 8 RECTIFIEI? VOLTAGE 3 souRcE INDICATOR l5 I MAXIMUM I SHUNT mxcxmc +25 ll cmcun I I II LINEAR |I g2 I AMPLIFIER I VOLTAGE l l I AMPLIFIER PCTURE CONTROLI SWITCH I l 4 2 I I .I HIGH PASS I FILTER I L X-RAY T TUBE 7 PULSE GATE FORMER f5 10 INDICATOR INTEGRATOR SWITCH 1 -15 rk zo I I I I l w? I I I SHUNT 1 n, CURRENT CONTROL L" I CURRENT U I so RCE fllg INVENTORS ATTORNEYS United States Patent U.S. Cl. 250-103 5 Claims ABSTRACT OF THE DISCLOSURE Apparatus for ascertaining tube current and plate voltage for an X-ray tube by which desired luminosity and maximum amount of information of a fluoroscopic picture can be obtained. For this purpose, the fluoroscopic picture is scanned by a signal forming unit such as a photocell whereby an electric signal is derived dependent on the content of the fluoroscopic picture. Such electric signal is, on the one hand, integrated by which a value characteristic of the all-over luminosity of the scanned picture portion is obtained and can be adjusted to a desired value by changing the tube current which is responsible for such luminosity. On the other hand, the electric signal is amplified and the effective value of the amplified signals is formed which has been found to be characteristic of the amount of details in the scanned portion of the fluoroscopic picture. The amplifier comprises a filter the transmission capacity of which increases with frequency which, in turn, is dependent on the number of details in the fluoroscopic picture. Thus, the aforesaid effective value will be the greater, the greater the number of details. Obviously, by altering the plate voltage which is responsible for the spectral energy distribution of the emitted radiation, plate voltages responsible for maximums of the effective value and, therewith, for maximum information will easily be ascertained.

This invention relates to an apparatus for ascertaining optimum values for tube current and anode voltage of an X-ray tube for X-ray examination on the basis of a fluoroscopic picture roduced by said X-ray tube.

As is known, in X-ray examination it is desired to obtain a maximum amount of information from the inner structure of an X-rayed body portion appearing on a fluoroscopic picture at a suitable brightness. The quality of a fluoroscopic picture produced by an X-ray tube is mainly determined by the tube current and the anode voltage employed and the obtension of maximum information depends on their adjustments to optimum values.

The intensity of X-radiation emerging from an X-ray tube which is responsible for the luminosity of the fluoroscopic picture is proportional to the tube current. Adjustment will be carried out dependent on the object to be examined, the focal distance of the X-ray tube and the efficiency of the apparatus for producing a fluoroscopic picture in such a manner that the mean luminosity permits an observation and evaluation of maximum and minimum dosage rates. Such adjustment is obtained by measuring the all-over luminosity of the fluoroscopic picture e.g. by means of a light meter and by adjusting the tube current so that the observed luminosity assumes a preset value which is in correspondence with external circumstances such as room illumination. The value of allover luminosity, however, may comprise completely uncovered or completely covered areas of the surface of the fluoroscopic picture. Therefore, the value obtained e.g. by the light meter is characteristic of the whole surface of the fluoroscopic picture-regardless of the presence or absence of information content. Consequently, just the 3,492,483 Patented Jan. 27, 1970 areas of the fluoroscopic picture which alone would be of interest as regards X-ray examination are, generally, either overluminous or underilluminated.

The amount of detail in the information content proper of the fluoroscopic picture depends on the spectral energy distribut'on of radiation since the absorption coeflicients of the various X-rayed substances depend on the wave length of the absorbed energy or radiation provided that X-rays of similar intensity are used. Therefore, the wave length or spectral energy distribution of the X-radiation employed has always to be selected in accordance with the actual object under examination. Such selection means an adjustment of the anode voltage of the X-ray tube by which the spectrum of its radiation is determined. For selection purposes schedules have been drafted which show anode voltages to be selected for various cases and objects. If the actual object to be examined considerably differs from its model in the aforesaid schedule as regards general sizes or dimensions and composition, which is the case in general, it depends on the dexterity of the radiologist to ascertain suitable values of anode voltage needed for obtaining the maximum amount of information.

The invention aims at eliminating such deficiencies and at the provision of an apparatus by means of which optimum,values of tube current and anode voltage may be automatically or semi-automatically ascertained so that only portions containing information of the fluoroscopic picture produced by an X-ray tube are considered when selecting the tube current while the tube or anode voltage is adjusted so as to give the maximum amount of detail in such information. The invention is based on the principle that luminosity of a specified portion of a fluoroscopic picture is in direct proportion to the integral of an electric signal derived from such fluoroscopic picture in a photoelectric manner. If such electric signals are permitted to be integrated only if they are due to fluoroscopic picture portions with information content, the integrated value will be characteristic of the all-over luminosity only of such information content. On the other hand, it has been found and can be shown that the maximum amount of detail in the information content is proportional to the maximum value of the root-mean square or effective value of the electric signal which is derived by scanning a specified portion of the fluoroscopic picture produced by an X-ray tube anode the voltage of which has to be selected so as to obtain a maximum such value. The derived electric signals will preferably be amplified and their root-mean square value mentioned above will be formed by supplying such amplified electric signals to a unit suitable to form the effective value of a changing voltage signal such as a rectifier having an ohmic resistance in its input so as to drop the voltage of the incoming signals.

Distinguishing between uncovered and covered portions of the fluoroscopic picture and portions with information content is feasible by employing a high pass filter having a transmission capacity linearly increasing with frequency. Such filter will permit to pass only frequencies which are higher than a certain value so that small changes in the electric signal or changeless values thereof will not reach the unit in which the effective or root-mean square value of the electric signal derived from the fluoroscopic picture is formed. On the other hand, the richer the details the higher the frequency and the greater the amplification rate.

Accordingly, the apparatus according to the invention for ascertaining optimum values for tube current and anode voltage of an X-ray tube for X-ray examination on the basis of a fluoroscopic picture produced by said X-ray tube comprises a signal forming unit arranged for scanning a specified portion of the surface of and deriving an electric signal from said fluoroscopic picture dependent on its information content, an integrator unit connected with the output of said signal forming unit for integrating said electric signal so as to obtain an integrated electric signal at the output of said integrator unit characteristic of tube current responsible for the luminosity of said information content, a gate unit between said signal forming unit and said integrator unit for operating said integrator unit, a pulse forming unit for controlling said gate unit on the basis of said electric signal, an amplifier unit connected to the output of said signal forming unit and having an amplification factor increasing with frequency for amplifying said electric signal coming from said signal forming unit so as to obtain an amplified electric signal at the output of said amplifier unit, said electric signal being amplified the more the higher its frequency, and a rectifier unit connected to the output of said amplifier unit for forming a root-mean square value of said amplified electric signal so as to obtain a rectified electric signal of effective value at the output of said rectifier unit characteristic of anode voltage responsible for the spectral-energy distribution in and, thereby, for the amount of detail of said information content.

It will be seen that the apparatus according to the invention performs just the desired functions of adjusting both the tube current and the anode voltage in the desired manner.

Viz, the electric signal derived by the signal forming unit which scans a specified portion of the fluoroscopic picture will either be constant when completely uncovered or completely covered areas of the fluoroscopic picture are scanned, or it will change when there are informative areas which means a more or less frequent change in the amplitude of the derived electric signal. Since the integrator unit has a gate unit connected to its input and the gate unit is controlled by a pulse forming unit on the basis of said electric signal, a pulse will be formed only when there are changes in the incoming electric signal which, in turn, means that there is information. Thus, the gate unit will be opened by the pulse forming unit whenever the electric signals carry information so that the integrator unit will iptegrate only electric signals which are derived from areas of the fluoroscopic picture containing informa tion. Consequently, the optimum luminosity of the fluoroscopic picture will be adjusted according to the overall luminosity of areas of the fluoroscopic picture containing information which is one of the main purposes of the invention.

Furthermore, the electric signals are fed into the aforementioned high pass filter With its transmissive capacity increasing with frequency. Thus, the greater the number of details or the amount of particulars, the higher the frequency of the amplified electric signals, which means that at the output of the rectifier unit for forming the effective value of the incoming signals a root-mean square value is obtained which is the greater the greater the amount of detail in the information content of the scanned area of the fluoroscopic picture. Changing the anode voltage of the X-ray tube, the value associated with maximum information content is easily ascertained.

Further details of the invention will be described on the basis of the accompanying drawing which shows a block diagram of an exemplified embodiment of the apparatus according to the invention.

In the drawing reference character 1 designates an X- ray tube which is used to produce a fluoroscopic picture 2 with information content 2a and completely covered or uncovered areas 2b. A signal forming unit 3 such as a photoelectric cell or the scanning ray of a television picture pick-up tube is arranged for scanning the fluoroscopic picture 2 so as to derive an electric signal by means of such scanning. The output of the signal forming unit 3 is bifurcated. On the one hand, it is connected through a gate unit 5 to an integrator unit 6. On the other hand,

it is connected through an amplifier 7 with the input of a rectifier 8 of the root-mean square or eifective value type. The gate unit 5 is controlled by a pulse forming unit 10 which is connected with the signal forming unit 3 through the aforesaid amplifier 7 so as to receive amplified electric signals. In the instant case, the amplifier unit 7 consists of a linear amplifier 11 and of a high pass filter 12 the transmission capacity of which linearly increases with frequency.

Reference characters 14 and 15 designate indicator means connected to the outputs of the integrator unit 6 and the rectifier unit 8, respectively.

The output of the integrator unit 6 has a control unit 17 connected to it which is connected between a current generator 18 and the cathode of the X-ray tube 1. The control unit 17 consists in the instant case of a current regulator such as a potentiometer 19, and a range selector switch 20 known per se.

In a similar manner, the output of the rectifier 8 has, through a maximum tracking unit 21, a control unit 22 connected to its output which is, in turn, connected between a voltage generator 23 and the plate of the X- ray tube 1. The latter is again composed of a voltage regulator such as a potentiometer 25 and of a range switch 26.

In operation, the X-ray tube 1 produces, in a manner known per se, the fluorescopic picture 2 the information content 2a of which will be scanned by the signal forming unit 3. The derived electric signal frequently changes because of the numerous differences in luminosity of details in the information content 2a. This changing electric signal is fed, on the one hand, into the gate unit 5 and, on the other hand, into the amplifier 7.

Signal voltages are supplied to the pulse forming unit 10 which forms an impulse thereof fed into the gate unit 5 to open it. In case of the completely uncovered area 2b of the fluoroscopic picture or with information content having few details the frequency of the formed electric signals is too low to pass the high pass filter 12. Consequently, the gate unit 5 will be opened, the integrator unit 6 will be operated and an integrated electric signal characteristic of the luminosity of a scanned area of the fluoroscopic picture 2 will be obtained only if such area is rich in information. The integrated signal proper is characteristic of the tube current which is responsible for the all-over luminosity of the scanned area such as 2a of the fluoroscopic picture 2.

In the instant case where the integrator unit 6 has a control unit 17 connected to its output which, in a manner known per se, is suitable for automatically changing the tube current of the X-ray tube 1 until a desired or preset value is obtained, the output signal of the integrator unit 6 causes the control unit 17 to change the current supplied by the current generator 18 until such value of the output signals appears.

On the other hand, the amplified electric signal of considerable frequency obtained at the output of amplifier 7 is supplied to the rectifier 8 for forming the effective or root-mean square value of such amplified signal. The properly rectified electric signal is supplied, on the one hand, into the indicator means and, on the other hand, into the maximum tracking unit 21. As is known, the maximum tracking unit 21 forms a differential of the incoming value and adjusts the control unit 22 and thereby the voltage supplied to the anode of the X-ray tube 1 until the differential of the effective value of the signal voltage becomes zero which means a maximum of the eifective voltage and, consequently, a maximum amount of particulars in the information content of the fluoroscopic picture 2. At the same time, the indicator means 15 shows a maximum deflection of its pointer.

It can be seen that the apparatus according to the invention permits a constant and automatic adjustment of the tube current to a preset or desired value and of the anode voltage to a value which is associated with a maximum content of information. The higher the frequency of the electric signal delivered by the signal forming unit, the greater the amplitude of the output signal of the amplifier 7 and, thus, the greater the eifective value of such signal delivered by the rectifier unit 8. Until the eifective value changes, the maximum tracking unit 21 operates the control unit 22 so as to change the anode voltage. When, however, the maximum tracking unit 21 finds a zero in the differential of the effective value associated with a maximum of the eifective value of the amplified electric signal, it stops the control unit 22 or practically causes it to play around a voltage value associated with maximum information.

However, the apparatus according to the invention could be used for ascertaining optimum values for tube current and anode voltage even without the control units 20 and 22 and the maximum tracking unit 21. In such case, the indicator means 14 and 15 serve as bases for ascertaining whether the tube current and the anode voltage are of optimum values. The tube current will be adjusted in a manner known per se until the indicator means 14 shows a desired value which may be associated with various illumination conditions of the ambiency. Likewise, the anode voltage is adjusted in a manner known per se until the indicator means 15 shows a maximum of the elfective value of the electric signal characteristic of the amount of detail in the information content 2a of the fluoroscopic picture 2.

An exemplified embodiment of the apparatus according to the invention has been described above, the amplifier unit 7 of which consisted of a special high pass filter 12 and a linear amplifier 11. Obviously, any other means could be employed which are suitable to withhold electric signals of little or no changes at all and to amplify them the more, the higher the frequency of change.

In the above described exemplified embodiment the pulse forming unit 10 was fed by the ouput signals of the amplifier unit 7. This is advantageous because, on the one hand, one amplifier unit suflices for the whole apparatus and, on the other hand, the pulse forming unit 10 is fed by preselected signals. Nevertheless, it would be possible to feed the pulse forming unit through a separate amplifier between the signal forming unit 3 and the pulse forming unit 10 or by the signal forming unit 3 directly. The range switches and 26 serve, in a manner known per se, for selecting the ranges of tube current and anode voltage, respectively, dependent on external circumstances such as the data of the X-ray tube 1 and the properties of the current generator 18 and the voltage generator 23, respectively.

What we claim is:

1. In apparatus for controlling the energy supply of an X-ray tube on the basis of a fluoroscopic image produced by said X-ray tube, a radiation-sensitive device for scanning a portion of and deriving an electric signal from said fluoroscopic image depending on its information content, and at least one control unit from the X-ray tube operated by an output signal from said radiation-sensitive device; the improvement comprising an amplifier unit including a high-pass filter connected to the output of said radiation-sensitive device, said filter having a transmission amplitude which is greater the higher the frequency, the output of said amplifier unit being connected to the input of said control unit.

2. Apparatus as claimed in claim 1, and an integrator unit connected with the output of said radiation-sensitive device for integrating said output signal so as to obtain an integrated electric signal at the output of said integrator unit characteristic of tube current responsible for the luminosity of said portion of said fluoroscopic image, said control unit including a rectifier unit connected to the output of said amplifier unit for forming an effective or root-mean square value of the amplified electric signal so as to obtain a rectified electric signal or eifective value at the output of said rectifier unit characteristic of tube voltage responsible for the spectral-energy distribution of said portion of said fluoroscopic image, and indicator means connected at the output of said integrator unit and said rectifier unit.

3. Apparatus as claimed in claim 1, said amplifier unit being composed of a linear amplifier and a high-pass filter having a transmission capacity linearly increasing with frequency.

4. Apparatus as claimed in claim 1, and an integrator unit connected to the output of said radiation-sensitive device for integrating said electric signal so as to obtain an integrated electric signal at the output of said integrator unit characteristic of tube current responsible for the luminosity of said portion of said fluoroscopic unit, said integrator unit having a control unit connected to its output for operation by said integrated electric signal, said last-named control unit being connected to the cathode of said X-ray tube and being connectible to a current generator for adjusting the cathode current of the X-ray tube in dependence on said integrated electric signal so as to obtain a cathode current suitable to yield a desired luminosity of said portion of said fluoroscopic image.

5. Apparatus as claimed in claim 1, said control unit including a rectifier unit connected to the output of said amplifier unit for forming an eifective or root-mean square value of said amplified electric signal so as to obtain a rectified electric signal or effective value at the output of said rectifier unit characteristic of tube voltage responsible for the spectral-energy distribution of said portion of said fluoroscopic image, said rectifier unit having a maximum tracking unit connected to its output to be operated by said rectified electric signal, said maximum tracking unit having a control unit connected to its output and connected to the anode of said X-ray tube and connectible to a voltage generator for adjusting the anode voltage of the X-ray tube dependent on the output signal of said maximum tracking unit so as to obtain maximum value of said rectified electric signal and the maximum amount of detail of said portion of said fluoroscopic image.

References Cited UNITED STATES PATENTS 2,962,594 11/1960 Duffy 1/1968 Markow 25083.3

US. Cl. X.R. 250-715, 95 

